Aromatic Compounds - BRIEF NOTES

Seite 1

Benzene, naming and its structure

LIQUID at room temperature and a key ingredient added to gasoline

Molecular formula = C6H6

Natural constituent of crude oil

Colourless + flammable liquid

Has CARCINOGENIC PROPERTIES

Used in chemical synthesis, e.g. plastics, drugs, dyes, explosives and pesticides

KEKULÉ'S MODEL OF BENZENE STRUCTURE = proposed a cyclic structure of benzene, with ALTERNATING single and double bonds

Suggests 3 alternating pi bonds were present, and these bonds were LOCALISED, formed from the sideways overlap of p-orbitals

THE DELOCALISED MODEL OF BENZENE = benzene is a planar cyclic hexagonal hydrocarbon, containing 6 carbon and 6 hydrogen atoms

Each carbon atom in benzene contains 4 outer shell electrons. For each carbon atom, 3 of its outer shell electrons are used to form 3 sigma bonds (with 2 other carbons and 1 hydrogen)

The 4th outer shell electron occupies a P-ORBITAL, which is at RIGHT ANGLES to the plane of the benzene ring

The 6 p-orbitals OVERLAP SIDEWAYS, in both directions ABOVE and BELOW the plane of the ring, forming 1 CONTINUOUS PI SYSTEM of 6 pi electrons, above and below the plane of the ring

These pi electrons are DELOCALISED as they can move freely in the ring and are not fixed between two carbon atoms only - known as RESONANCE (gives rise to benzene's usual stability)

EVIDENCE AGAINST KEKULE'S MODEL, AND FOR THE DELOCALISED MODEL:

Benzene doesn't undergo electrophilic addition reactions and doesn't react readily with bromine water. So, benzene has a LOWER CHEMICAL ACTIVITY THAN PREDICTED BASED ON KEKULE'S MODEL

The enthalpy of hydrogenation of benzene was LOWER than expected. This suggests STRUCTURAL ASIMILARITIES than if cyclohexene had 3 double bonds (cyclohexa-1,3,5-triene). This means it needs less hydrogen, suggesting hydrogen can't just break the double bonds and form 'cyclohexane'

All the carbon-carbon bonds in benzene are the same length. This was revealsed from X-RAY CRYSTALLOGRAPHY studies into the structure of benzene, disputing what would be suggested from kekule's model with alternating single and double bonds.

Aromatic compounds are VERY STABLe, due to their delocalised pi electrons

NAMING:

MONOSUBSTITUTED AROMATIC COMPOUNDS: When a halogen is involved, always name FROM THE HALOGEN eg/ priority over methyl

DISUBSTITUTED AROMATIC COMPOUNDS: the first side chain in the alphabet is always given the LOWEST number possible

TRISUBSTITUTED AROMATIC COMPOUNDS: if you don't get the lowest numbers from naming from the halogen, then DON'T. Name from wherever will give the lowest possible combinations of numbers. When the lowest number combinations are all the same eg. groups on 1 3 and 5, then choose the numbering system that gives the lowest number to the FIRST side chain in the alphabet eg. 1-chloro-3-iodo-5-nitrobenzene

In some aromatic compounds, the main functional group is NOT the aromatic ring. If a hydrogen atom is removed from the actual functional group, and replaced with a benzene ring, this ring is now called PHENYL eg. phenylamine, phenylethene, 3-phenylpropan-1-ol

Seite 2

Electrophilic Substitution reactions of benzene

NITRATION:

Reagents: CONCENTRATED nitric acid, in the presence of a concentrated sulfuric acid catalyst

Mechanism: Electrohilic Substitution

Electrophile involved: NO2+ (formed from HNO3 reacting with catalyst H2SO4)

Conditions: Reflux at 50-60 degrees. On higher temperatures, disubstitution can occur so a second nitro group is added If the bezene ring already has a side group eg. methyl, then the nitro group can also join at different positions

The mechanism forms a H+, which can react with the HSO4- formed from generating the electrophile, to reform H2SO4 catalyst

Reaction equation: WATER IS ALSO FORMED!!!

HALOGENATION:

Halogens won't react benzene unless a HALOGEN CARRIER CATALYST IS USED. This is because, due to benzene having a delocalised pi system and alkenes having a localised pi system, benzene has a LOWER ELECTRON DENSITY than alkenes. So, it polarises halogens LESS, and induces a weaker dipole in the halogens than an alkene would

Reagents: bromine, halogen carrier catalyst (FeBr3, AlBr3, FeCl3, AlCl3)

Mechanism: electrophilic substitution

Electrohile involved: Br+ or Cl+

Conditions: room temoperature and pressure

Halogen carrier catalust produces the electrophile, and is later reformed!

Reaction equation: HBr is also formed!!!/ HCl

FRIEDAL CRAFTS ALKYLATION:

Reagents: any CHLOROALKANE (haloalkane), in the presence of an anhydrous AlCl3 halogen carrier catalyst

Mechanism: Electrophilic Substitution

Electrophile involved eg. if adding an ethyl group: CH3CH2+

Conditions: room temperature and pressure

Eg. forming electrophile: AlCl3 + CH3CH3Cl ---> AlCl4- + CH3CH2+

Reaction equation: HCl also formed!!

FRIEDAL CRAFTS ACYLATION:

When benzene reacts with an acyl chloride, in the presence of a halogen carrier catalyst, an AROMATIC KETONE is formed.

Reagents: an acyl chloride, in the presence of anhydrous AlCl3 halogen carrier catalyst

Mechanism: Electrophilic Substitution

Electrophile involved eg. CH3CO+

Conditions: heat under reflux at 60 degreesm anhydrous conditions (to avoid the acyl chloride turning into a carboxylic acid)

Forms eg. phenylethanone (forms PHENYL KETONES - how you name)

Reaction equation: HCl is also formed!!!

Seite 3

Phenol

Molecular formula = C6H5OH

Phenols are WEAKLY ACIDIC. They are WEAKER acids than carboxylic acids

BOTH carboxylic acids and phenols will react with SODIUM METAL and SODIUM HYDROXIDE but ONLY carboxylic acids will react with SODIUM CARBONATE, as a phenol is NOT A STRONG ENOUGH ACID TO REACT

Since phenol is a very WEAK ACID, it cannot react with weak bases (CARBONATES = WEAK BASES)

Sodium hydroxide = A STRONG BASE so phenol CAN react with it

ALCOHOLS ARE LESS ACIDIC THAN PHENOLS

Order of acidity alcohols<phenols<carboxylic acids

When phenols partially dissociate (weak acids), they form the PHENOXIDE ION (benzene ring attached to a negatively charged oxygen, as the hydrogen has left) This phenoxide ion can form SALTS

When drawing phenoxide salts = include the O- charge and the charge of the metal ion eg. benzene ring attached to O- Na+

Distinguish between alcohols, phenols and carboxylic acids = react with sodium hydroxide eliminates alcohol, react with carbonate elimates phenol. Or, by comparing the Ka equilibrium constant

Phenols are used in the production of plastics, antiseptics, disinfectants and resins for paints

REACTION WITH SODIUM:

Phenols react with sodium metal to form SODIUM PHENOXIDE SALT + 1/2 H2 (1 H is lost but H comes in H2 so say 1/2 H2)

Phenols react with sodium hydroxide to form the salt SODIUM PHENOXIDE and water (neutralisation reaction - acid + base ---> salt + water)

The sodium phenoxide compound is MORE SOLUBLE than the original phenol. So, the solid phenol DISSOLVES on addition of NaOH

REACTION WITH BROMINE:

Reagents = Br2

Conditions = room temperature and pressure, no halogen carrier catalyst

Reaction equation: HBr forms too!!!

The product is 2,5,6-TRIOBROMOPHENOL. This is a WHITE SOLID!!!!!!!!!!!!!!! (MUST REMEMBER WHITE SOLID!!!!)

So, phenol undergoes MULTIPLE SUBSTITUTION (TRISUBSTITUTION) whereas benzene will only undergo MONOSUBSTITUTION

REACTUON WITH NITRIC ACID:

Reagents = DILUTE nitric acid

Conditions = room temperature and pressure

Only MONOSUBSTITUTION will occur as the nitro group is a DEACTIVATING GROUP

This forms a mixture of either 4-nitrophenol or 2-nitrophenol (as OH on phenol is a 2,4 directing group)

WHY PHENOL IS MORE REACTIVE THAN BENZENE:

Benzene and phenol both contain delocalised pi electrons

However, the lone pair on the oxygen from the hydroxyl group on phenol is DELOCALISED INTO THE RING

The hydroxyl group can therefore ACTIVATE the aromatic ring, which INCREASES its electron density

As a result, phenol is able to INDUCE A DIPOLE in bromine, without the use of a halogen carrier and is therefore MORE SUSCEPTIBLE TO ELECTROPHILIC ATTACK

However, bezene CANNOT induce a dipole in bromine as it has a LOWER ELECTRON DENSITY THAN PHENOL. Therefore, a halogen carrier is required to produce a stronger electrophile

As phenol is more reactive than benzene + more susceptible to electrophilic attack, nitration of phenol, unlike benzene, doesn't need reflux (done at RTP), doesn't require a concentrated sulfuric acid catalyst and dilute nitric acid can be used instead of concentrated nitric acid

MONONITRATION takes place with nitration of a phenol, and not trisubstitution, like with halogens, as the nitro group is a deactivating, electron withdrawing group, so means the aromatic ring is less susceptible to electrophilic attack after 1 nitro group is added

HOW IS PHENOL PARTIALLY SOLUBLE IN WATER?

The hydroxyl group in phenol can form hydrogen bonds with water molecules. The aromatic ring is, however, non-polar and CAN'T form hydrogen bonds with water molecules, so, phenol is PARTIALLY SOLUBLE in water.

Seite 4

Purification of an organic solid steps:

Filter the impure crystals out of solution, under a reduced pressure

Wash the crystals with cold water

Impure solid then needs to be recrystallised:

Find a suitable solvent that the impurities will dissolve in or the pure product but NOT BOTH

Dissolve the impure solid in a MINIUMUM VOLUME of HOT SOLVENT

Once the solid has dissolved, cool the solution down in an ice water bath

The impurities should remain in solution whilst the pure organic solid precipitates out, or vice vera

If impurities are precipitated out and pure product still in solvent, remove the impurities and cool the hot solvent to crystallise the desired product from solution, then filter it out

Leave the crystals to dry off

TO CHECK PURITY = the melting point can be checked with a melting point apparatus and compared to known melting point value of that compound

Directing groups in aromatic compounds

DIRECTING EFFECT = how a functional group that is attached directly to an aromatic ring affects WHICH aromatic hydrogen atoms are more likely to undergo electrophilic substitution

Positions on the benzene ring = 1, 2 position 2's, 2 position 3's, 1 position 4 (look at diagram in folder)

ACTIVATING GROUPS = -NH2 and -OH

DEACTIVATING GROUP = -NO2

2,4 directing groups

-NH2 and -OH are 2,4 DIRECTING GROUPS!

The lone pair of electrons on the oxygen/nitrogen is delocalised into the aromatic ring

This delocalisation ACTIVATES the ring at the 2 and 4 positions only

This makes the 2 and 4 positions more susceptible to electrophilic attack

The hydroxyl/amine groups are therfore 2 and 4 directing groups! (remember this can give rise to trisubstitution as there are 2 position 2's)

3 directing groups

-NO2 is a 3 directing group

So, the nitro group will activate the ring at the 3 position only. This makes the aromatic ring susceptible to electrophilic attack only at the 3 position

This is due to the ELECTRON WITHDRAWING EFFECT of the nitro group , which withdraws pi electrons from the aromatic ring, towards itself. This makes the aromatic ring electron rich only at the 3-position

DIBROMINATION (disubstitution) CANNOT take place as the nitro group is a DEACTIVATING GROUP. This means the nitro group REDUCES the electron density of the aromatic ring by drawing the pi electrons from the aromatic ring, towards itself. This means the aromatic ring is less susceptible to MULTIPLE electrophilic attacks!

NOTES:

If you need to predict the relative quantities of product produced from 2,4 directing groups, more of the 2-directing group product will be formed than the 4-directing group product as there are 2 2 positions but only 1 4 position

When you are given an organic product to plan an organic synthesis for, and there are multiple steps, consider the positions of the groups attached to the benzene ring and this will help you decide which group should be attached first, based on their directing groups.

Eg. if NO2 is involved and the other group is not at the 3 position, then the other group should be added first because NO2 can only direct groups to attach at the 3 position